An organic EL device, which has an organic thin film layer including a light emitting layer between an anode and a cathode and which emits light from an exciton energy resulted from the recombination of holes and electrons injected into the light emitting layer, has been known.
Sine the organic EL device is a spontaneous emitting device, it has been expected to be applicable, using its advantages, to a light emitting device with high current efficiency, high image quality, low power consumption and wide design freedom for thinner products. The organic EL device has been still required to be further improved in its properties, for example, in the current efficiency.
In this regard, to enhance the internal quantum efficiency, a light emitting material (phosphorescent material) which emits light from triplet exciton has been developed, and a phosphorescent organic EL device are reported in recent years.
By forming a light emitting layer (phosphorescent layer) using the above phosphorescent material, an internal quantum efficiency of 75% or more, theoretically about 100% is obtained, to realize an organic EL device having high efficiency and low power consumption. Further, a doping method in which a light emitting material is doped as a dopant into a host material for forming a light emitting layer is known.
In a doped light emitting layer, excitons can be efficiently generated from charges injected into a host material. The exciton energy of generated excitons is transferred to a dopant, and this allows the dopant to emit light in high efficiency.
To intermolecularly transfer the energy from a host material to a phosphorescent dopant, the excited triplet energy Eg(T) of the host material has to be larger than the excited triplet energy Eg(S) of the phosphorescent dopant.
CBP (4,4′-bis(N-carbazolyl)biphenyl) is a well known material which has an effectively large excited triplet energy (Patent Document 1).
If CBP is used as a host material, the energy can be transferred to a phosphorescent dopant which emits light with a specific wavelength (for example, green and red), and an organic EL device having high efficiency can be obtained.
When CBP is used as a host material, the current efficiency is drastically enhanced by phosphorescent emission on one hand, but the lifetime is very short to make the device unsuitable for practical use on the other hand.
This may be because that CBP has a molecular structure less resistant to oxidation and therefore its molecule is largely degraded by holes.
Patent Document 2 discloses a technique in which a condensed ring derivative having a nitrogen-containing ring such as carbazole is used as a host material for a red-emitting phosphorescent layer. This technique enables the improvement of current efficiency and lifetime, but is not satisfactory for practical application in some cases.
A wide variety of fluorescent host materials (fluorescent hosts) for a fluorescent dopant is known, and various host materials which can form, in combination with a fluorescent dopant, a fluorescent layer excellent in current efficiency and lifetime are proposed.
The excited singlet energy Eg(S) of a fluorescent host is larger than that of a fluorescent dopant, but its excited triplet energy Eg(T) is not necessarily large. Therefore, the fluorescent host cannot be simply used as a host material (phosphorescent host) for a phosphorescent layer.
For example, an anthracene derivative is well known as a fluorescent host. However, the excited triplet energy Eg(T) of anthracene derivative is as relatively small as about 1.9 eV. Therefore, the energy transfer to a phosphorescent dopant having an emission wavelength in a visible light region of 500 to 720 nm can not be secured. Further, the anthracene derivative cannot confine the excited triplet energy within a light emitting layer.
Therefore, the anthracene derivative is unsuitable as a phosphorescent host.
Further, perylene derivatives, pyrene derivatives and naphthacene derivatives are not preferred as a phosphorescent host for the same reason.
Patent Document 3 proposes to use an aromatic hydrocarbon compound as a phosphorescent host, which has a central benzene skeleton having two aromatic substituents at its meta positions.
However, the aromatic hydrocarbon compound described in Patent Document 3 has a highly symmetric, rigid molecular structure composed of five aromatic rings which are arranged bilaterally symmetrically with respect to the central benzene skeleton. Therefore, the light emitting layer would be likely to crystallize.
Patent Documents 4 to 6 disclose organic EL devices each employing an aromatic hydrocarbon compound, and Patent Documents 7 to 9 disclose organic EL devices each employing a fluorene compound. However, these documents are completely silent about the effectiveness of these compounds as a phosphorescent host.
Patent Documents 10 to 15 describe devices which employ a phosphorescent host material comprising a fluorene compound, Patent Documents 11, 13 and 15 disclose divalent fluorene compounds characterized by benzene rings directly bonded to both ends of the fluorene ring, and Patent Document 12 discloses a compound having a fluorene structure having an aryl group at its 9-position. However, the current efficiency and the device lifetime are not satisfactory even if these compounds are used as a phosphorescent host material.
Patent Documents 8 and 16 disclose hydrocarbon compounds having a condensed polycyclic aromatic ring and a fluorene ring which are directly bonded to each other. However, these documents are completely silent about the effectiveness of an organic EL device employing these compounds in combination with a phosphorescent material. In addition, a perylene ring and a pyrene ring, which are not suitable for use in a light emitting layer of phosphorescent device because of their low triplet energy level, are exemplified in these documents as the condensed polycyclic aromatic ring. Namely, these documents fail to teach materials which are effective for a phosphorescent device.
Patent Document 17 discloses a host material for phosphorescent device which essentially includes two or more fluorene rings and a naphthalene ring. Patent Document 18 discloses a material in which a divalent fluorene is bonded to a substituted phenanthrene ring and a benzene ring or other condensed polycyclic aromatic rings.